Field of the Invention
This present invention relates to a method of regeneration of mixed metal oxide catalyst. In particular, the present invention is directed to the regeneration of bismuth-moly-iron-promoted mixed metal oxide catalyst useful in the ammoxidation of propylene to acrylonitrile.
Mixed metal oxide based supported catalysts which are active in ammoxidation reactions, particularly in the production of acrylonitrile or methacrylonitrile from propylene or isobutylene, ammonia and oxygen are known in the art. A typical mixed metal oxide catalyst suitable for ammoxidation contains molybdenum, bismuth and iron promoted with various metals. It has been observed that the activity of such catalyst declines as a result of prolonged exposure to ammoxidation conditions and this decline, which manifests itself as a reduction in the yield of useful reaction products given by the catalyst, is accompanied by a partial loss of molybdenum. Typically, when the yield of acrylonitrile with respect to propylene conversion is reduced by five or more percentage units compared with the value of a fresh catalyst, the ammoxidation catalyst is spent and needs regeneration.
Several methods have been proposed in the art for the regeneration of spent ammoxidation catalyst. For example, see U.S. Pat. Nos. 3,882,159; 4,052,332; 4,757,038; 4,536,483; and 4,425,255. Each of these patents discloses various means of regeneration of ammoxidation catalyst. In addition, German Patent No. 3,311,521 also discloses a regeneration procedure for ammoxidation catalyst. Of the foregoing patents, U.S. Pat. Nos. 4,425,255, 4,609,635 and 4,052,332, as well as German Patent 3,311,521 each are concerned with regeneration of bismith-molybdenum-iron-containing catalyst promoted with various other elements. U.S. Pat. No. 3,882,159 discloses a regeneration procedure which includes adding molybdenum oxide on an inert support to the catalyst for regeneration. U.S. Pat. No. 4,609,635 discloses that the catalyst may be regenerated using aqueous ammonium molybdate, in particular, ammonium heptamolybdate to add molybdenum to the catalyst. A similar teaching with the addition of molybdenum trioxide and ammonium heptamolybdate can be seen in German Patent 3,311,521 as well as the U.S. Pat. No. 4,425,255. Generally, each of the references discloses that the regeneration of the promoted bismuth, iron, molybdenum catalyst system requires the replacement of molybdenum onto the catalyst.